1. Field of the Invention
The present invention relates to analog-to-digital converters (ADCs), and more particularly, to a voltage reference generator with negative feedback for use in establishing reference voltages for ADCs.
2. Description of the Related Art
Switched capacitor ADCs provide efficient high speed analog-to-digital signal conversion. A representative switched capacitor ADC 10 is shown in FIG. 1, in the form of a multi-stage pipelined ADC. As shown, ADC 10 includes multiple stages, such as stages 11 and 12, each providing one or more bits of digital data to a digital correction circuit 15, which resolves the digital output from each stage into an overall digital output 16 corresponding to an analog input 17. Each stage is a switched capacitor circuit operating in response to clock signals such as phi 1 and phi 2 and comparing an analog voltage input to thresholds based on reference signals Vrefp and Vrefn, to produce digital output.
For proper operation of ADC 10, generators are needed for phase and timing signals as well as for reference voltages, as shown respectively at 20 and 30 of FIG. 1. Thus, generator 20 for phase and timing signals generates clock signal phi 1 for use during the sample phase of multiple stages 11 and 12, as well as clock signal phi 2 for use during the amplification phase of multiple stages 11 and 12. Likewise, generator 30 generates reference voltages Vrefp and Vrefn for use by multiple stages 11 and 12. The design of the present application applies to the generator 30 for the reference voltages.
FIG. 2 shows a conventional generator 30 for generating reference voltage Vrefp, with a similar circuit, shown schematically at 31, to generate reference voltage Vrefn. As shown in FIG. 2, generator 30 includes a source follower 32 connected between voltage source V+ and a current source 35 which, in turn, is connected to ground. Source follower 32 is driven at its gate side by amplifier 34, connected via negative feedback using a reference voltage Vref as a reference and the output Vrefp as negative feedback.
With this arrangement, source follower 32 is driven by amplifier 34 to provide output Vrefp with good current capabilities stabilized through negative feedback at a voltage level corresponding to Vref.
However, in use of generator 30 shown in FIG. 2, for example, due to higher frequency switching of generator 30, and due to noise/glitches generated by ADCs, the amplifier 34 (FIG. 2) must respond promptly, reacting quickly to recovery Vrefp to an ideal value to avoid noise (e.g., preferably within a fraction of a clock period). However, this is difficult to achieve for high speed ADCs. An alternative is to use an external capacitor CEXT (e.g., with a sufficiently large capacitance) to lower the impedance seen by the reference at high frequencies. This alternative may minimize switching glitches and noise, but it also requires extra circuitry, and for example, an extra pin.
Another conventional reference voltage generator is shown in FIG. 3. As shown, the generator uses an operational amplifier (OP-AMP) 40, connected via negative feedback through its output of the OP-AMP.
Although the generator in FIG. 3 requires no external capacitor, and can be designed for use with high bandwidth applications, the OP-AMP requires a large power supply, and the circuit area is large.